1. Field of the Invention
The present invention relates to portable equipment such as a portable radio or telephone for transmitting and receiving information using electromagnetic wave.
2. Description of the Prior Art
In conventional portable radios, an external electromagnetic wave influence causes an internal system such as a transmitting unit and a receiving unit to malfunction and to deteriorate a transmitting performance. Therefore, in order to avoid such a problem, there is provided an electric shield of a metallic nature in a conducting body of the portable radio equipment so as to cut off the external electromagnetic wave influence.
However, it is confirmed by the inventors of the present invention that a high-frequency current flowing through the electric shield causes an adverse effect on a radiation pattern which is radiated from an antenna of the portable radio. It is presumed that a vertically polarized wave among the electromagnetic wave is relatively large and thus it is likely to be favorable that a gain of the vertically polarized wave in the vertical direction is large.
An example of the conventional portable radio equipment where the electric shield is provided is shown in FIG. 1. In the same figure, the reference numeral 102 shows a housing serving as the electric shield, and the reference numeral 103 shows an antenna. A simulation for the radiation characteristics of the electromagnetic wave in the radio-frequency of an L band is carried out using a model of the portable radio shown in FIG. 1. With reference to FIG. 1, dimensions for the housing 102 are that a width thereof is 0.4 .lambda., a depth 0.15 .lambda. and a height 0.5 .lambda., where .lambda. indicates a wavelength. The simulation is carried out using a small-sized model which is equipped with a .lambda./4-monopole antenna and whose antenna can interface with a feeder without a matching circuit. A result of the simulation is shown in FIG. 2.
FIG. 2 shows the calculated result of the radiation pattern of the vertically polarized wave around the antenna being placed in the center with respect to (A) x-z plane, (B) x-y plane and (C) y-z plane. As shown in FIG. 1, an x coordinate is placed in a width direction, a y coordinate is in a depth direction and a z coordinate is in a parallel direction to an axis of the .lambda./4-monopole antenna. The electromagnetic simulator for arbitrary models is made on a super-computer employing a spatial network method. An electromagnetic field in the vicinity of the model was calculated by applying the simulator to an ordinary portable telephone model for the L band. A parameter for three dimensional lattice network is 80.times.70.times.90 (.DELTA.d), where a unit length of the lattice, .DELTA.d, is .lambda./40. A far-field radiation pattern is calculated from the electromagnetic field on a surface of a closed-area over the model.
Referring to a result of the simulation pattern, the radiation pattern for the x-y plane (B) which shows a pattern for a cross section vertical to the antenna is omnidirectional (radiate the same in all directions). On the other hand, in the radiation pattern with respect to the y-z plane (C), a maximum radiation direction is indicated at approximately 50 degrees tilted from a y axis against a z axis and in a negative z-axis direction. As a result of normalization by a maximum radiation gain (a normalized pattern is a dimesionless number with a maximum value of unity), the radiation pattern with respect to the y-z plane (C) indicates a characteristic of deterioration by approximately 5 dB from the maximum radiation gain, compared to the maximum radiation gain on the x-z plane.
FIG. 3 shows respective radiation patterns which are theoretically optimum, corresponding to FIG. 2.
However, in a .lambda./2-dipole antenna, the maximum radiation direction shall theoretically lie in 90 degrees from an antenna axis in the plane including the antenna axis. Therefore, the fact that the maximum radiation direction is deflected as observed in the radiation pattern of the y-z pattern (C) as the above simulation result demonstrates that the radiation pattern of the antenna itself is affected and disturbed by the radio-frequency current flowing through the electrically shielded housing 102.
Since a large radio-frequency current may flow through the housing due to current distribution of the antenna itself, the radiation pattern is much affected in the .lambda./4-monopole antenna. When the height of housing is converted to a corresponding electrical length and the converted electrical length is approximately equal to the electromagnetic wavelength, a current whose phase is opposite to the radio-frequency current flows on the antenna. As a result, the radiation pattern in the horizontal direction in the portable radio is cancelled out against each other, thus causing to deteriorate the radiation gain in the horizontal direction. In this connection, when the portable radio is designed, without considering an effect of the housing, by calculating the radiation pattern with respect to the antenna alone, a desired electromagnetic radiation pattern can not be obtained because of the influence of the housing even if the portable radio is designed such that the maximum radiation shall be obtained in 90 degrees against the antenna axis.
FIG. 4 shows a model of a portable radio equipment employing an inverted F antenna. In the same figure, the reference numeral 103 designates an antenna, the reference numeral 106 is a short-circuit wire, and the reference numeral 106 designates a feeder for a signal. FIG. 5 shows a calculated result of a radiation gain pattern for the model. In this case, too, a maximum radiation direction in a radiation pattern of a (C) y-z plane is deflected from the horizontal direction (y-axis direction), thus indicating that the radio-frequency current flowing through the housing affects to deteriorate the radiation pattern.
As described above, though the electric shield is provided to cut off the influence by the external electromagnetic wave, conventionally there exists a problem where the radiation characteristic of antenna alone is disturbed by the radio-frequency current flowing through the housing and thus the desirable radiation characteristic for the portable radio can not be obtained.